/*
 * Copyright (C) 2009 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.collect;

import com.google.common.annotations.GwtCompatible;
import com.google.common.primitives.Booleans;
import com.google.common.primitives.Ints;
import com.google.common.primitives.Longs;
import java.util.Comparator;
import javax.annotation.Nullable;

/**
 * A utility for performing a chained comparison statement. For example:
 * 
 * <pre>
 *    {@code
 *
 *   public int compareTo(Foo that) {
 *     return ComparisonChain.start()
 *         .compare(this.aString, that.aString)
 *         .compare(this.anInt, that.anInt)
 *         .compare(this.anEnum, that.anEnum, Ordering.natural().nullsLast())
 *         .result();
 *   }}
 * </pre>
 *
 * <p>
 * The value of this expression will have the same sign as the <i>first nonzero</i> comparison
 * result in the chain, or will be zero if every comparison result was zero.
 *
 * <p>
 * <b>Note:</b> {@code ComparisonChain} instances are <b>immutable</b>. For this utility to work
 * correctly, calls must be chained as illustrated above.
 *
 * <p>
 * Performance note: Even though the {@code ComparisonChain} caller always invokes its
 * {@code compare} methods unconditionally, the {@code
 * ComparisonChain} implementation stops calling its inputs' {@link Comparable#compareTo compareTo}
 * and {@link Comparator#compare compare} methods as soon as one of them returns a nonzero result.
 * This optimization is typically important only in the presence of expensive {@code compareTo} and
 * {@code compare} implementations.
 *
 * <p>
 * See the Guava User Guide article on
 * <a href= "https://github.com/google/guava/wiki/CommonObjectUtilitiesExplained#comparecompareto">
 * {@code ComparisonChain}</a>.
 *
 * @author Mark Davis
 * @author Kevin Bourrillion
 * @since 2.0
 */
@GwtCompatible
public abstract class ComparisonChain {
    private ComparisonChain() {}

    /**
     * Begins a new chained comparison statement. See example in the class documentation.
     */
    public static ComparisonChain start() {
        return ACTIVE;
    }

    private static final ComparisonChain ACTIVE = new ComparisonChain() {
        @SuppressWarnings("unchecked")
        @Override
        public ComparisonChain compare(Comparable left, Comparable right) {
            return classify(left.compareTo(right));
        }

        @Override
        public <T> ComparisonChain compare(@Nullable T left, @Nullable T right, Comparator<T> comparator) {
            return classify(comparator.compare(left, right));
        }

        @Override
        public ComparisonChain compare(int left, int right) {
            return classify(Ints.compare(left, right));
        }

        @Override
        public ComparisonChain compare(long left, long right) {
            return classify(Longs.compare(left, right));
        }

        @Override
        public ComparisonChain compare(float left, float right) {
            return classify(Float.compare(left, right));
        }

        @Override
        public ComparisonChain compare(double left, double right) {
            return classify(Double.compare(left, right));
        }

        @Override
        public ComparisonChain compareTrueFirst(boolean left, boolean right) {
            return classify(Booleans.compare(right, left)); // reversed
        }

        @Override
        public ComparisonChain compareFalseFirst(boolean left, boolean right) {
            return classify(Booleans.compare(left, right));
        }

        ComparisonChain classify(int result) {
            return (result < 0) ? LESS : (result > 0) ? GREATER : ACTIVE;
        }

        @Override
        public int result() {
            return 0;
        }
    };

    private static final ComparisonChain LESS = new InactiveComparisonChain(-1);

    private static final ComparisonChain GREATER = new InactiveComparisonChain(1);

    private static final class InactiveComparisonChain extends ComparisonChain {
        final int result;

        InactiveComparisonChain(int result) {
            this.result = result;
        }

        @Override
        public ComparisonChain compare(@Nullable Comparable left, @Nullable Comparable right) {
            return this;
        }

        @Override
        public <T> ComparisonChain compare(@Nullable T left, @Nullable T right, @Nullable Comparator<T> comparator) {
            return this;
        }

        @Override
        public ComparisonChain compare(int left, int right) {
            return this;
        }

        @Override
        public ComparisonChain compare(long left, long right) {
            return this;
        }

        @Override
        public ComparisonChain compare(float left, float right) {
            return this;
        }

        @Override
        public ComparisonChain compare(double left, double right) {
            return this;
        }

        @Override
        public ComparisonChain compareTrueFirst(boolean left, boolean right) {
            return this;
        }

        @Override
        public ComparisonChain compareFalseFirst(boolean left, boolean right) {
            return this;
        }

        @Override
        public int result() {
            return result;
        }
    }

    /**
     * Compares two comparable objects as specified by {@link Comparable#compareTo}, <i>if</i> the
     * result of this comparison chain has not already been determined.
     */
    public abstract ComparisonChain compare(Comparable<?> left, Comparable<?> right);

    /**
     * Compares two objects using a comparator, <i>if</i> the result of this comparison chain has
     * not already been determined.
     */
    public abstract <T> ComparisonChain compare(@Nullable T left, @Nullable T right, Comparator<T> comparator);

    /**
     * Compares two {@code int} values as specified by {@link Ints#compare}, <i>if</i> the result of
     * this comparison chain has not already been determined.
     */
    public abstract ComparisonChain compare(int left, int right);

    /**
     * Compares two {@code long} values as specified by {@link Longs#compare}, <i>if</i> the result
     * of this comparison chain has not already been determined.
     */
    public abstract ComparisonChain compare(long left, long right);

    /**
     * Compares two {@code float} values as specified by {@link Float#compare}, <i>if</i> the result
     * of this comparison chain has not already been determined.
     */
    public abstract ComparisonChain compare(float left, float right);

    /**
     * Compares two {@code double} values as specified by {@link Double#compare}, <i>if</i> the
     * result of this comparison chain has not already been determined.
     */
    public abstract ComparisonChain compare(double left, double right);

    /**
     * Discouraged synonym for {@link #compareFalseFirst}.
     *
     * @deprecated Use {@link #compareFalseFirst}; or, if the parameters passed are being either
     *             negated or reversed, undo the negation or reversal and use
     *             {@link #compareTrueFirst}.
     * @since 19.0
     */
    @Deprecated
    public final ComparisonChain compare(Boolean left, Boolean right) {
        return compareFalseFirst(left, right);
    }

    /**
     * Compares two {@code boolean} values, considering {@code true} to be less than {@code false},
     * <i>if</i> the result of this comparison chain has not already been determined.
     *
     * @since 12.0
     */
    public abstract ComparisonChain compareTrueFirst(boolean left, boolean right);

    /**
     * Compares two {@code boolean} values, considering {@code false} to be less than {@code true},
     * <i>if</i> the result of this comparison chain has not already been determined.
     *
     * @since 12.0 (present as {@code compare} since 2.0)
     */
    public abstract ComparisonChain compareFalseFirst(boolean left, boolean right);

    /**
     * Ends this comparison chain and returns its result: a value having the same sign as the first
     * nonzero comparison result in the chain, or zero if every result was zero.
     */
    public abstract int result();
}
